1. Field of the Invention
The present invention relates to low profile radial tires for trucks and buses wherein steel cords are used for carcass plies as reinforcing elements for tires and more particularly to an improvement in the construction of the steel cords.
2. Prior Art
Heretofore, there have been used the steel cords having the structure of 2+7 as shown in FIG. 3, 3+9 in FIG. 4 or 1.times.12 in FIG. 5.
Steel cords 3 of 2+7 in FIG. 3, however, having a core 6 composed of two filaments 4 twisted together as shown in the figures, the alignment of the filaments becomes vertical at one position in the lengthwise direction of the cords as shown in FIG. 3(B), while the alignment becomes lateral at another position as shown in FIG. 3(A). These alignments appear at every 1/4 pitch. It results in that said cords are composed of different portions having a different flexural rigidity in the lengthwise direction of the cords. When said cords are applied to a tire, the portions in a lateral alignment having a lower flexural rigidity are subjected to a concentrated fatigue breakage. Furthermore, outer filaments 5 which form an outer layer 7 are apt to close to a center over a circumscribed circle of the core 6 as shown in the figures by a solid line compared with a dotted line which illustrates an expected position. Accordingly, the difference of the flexural rigidity between the vertical alignment portions and the lateral alignment portions becomes larger, so there is a serious problem in the fatigue resistance. In addition, all filaments of said cords being covered by rubber, residual air which is caught during tire formation is hardly passed away to the outside through the inside of the cords. Then a tire is manufactured with remaining air in the cords, so that the tire manufactured is subjected to an early failure due to the residual air.
Steel cords 3 of 3+9 as shown in FIG. 4 having a narrow space between adjacent outer filaments 5, the penetration of rubber into the cords is insufficient, so that the outer filaments 5 contact with each other and wear out. It results in the deterioration of fatigue resistance.
Steel cords 3 of 1.times.12 as shown in FIG. 5 are composed of 12 filaments having a same diameter which are twisted in the same direction with the same pitch, so the outer filaments 5 fit in surface depressions of a core strand 6 and thereby the penetration of rubber is hindered. Consequently, core filaments 4 are apt to be drawn out during a tire service.
As mentioned above, prior art steel cords have not been provided with balanced properties between the uniform flexural rigidity and the rubber penetration property.
On the other hands, it has been a recent trend to use low profile tires having a low aspect ratio, namely wider tires with a flatter profile. The low profile tires are apt to cause a great deflection at a shoulder portion when they are travelled with a reduced inner pressure, so that steel carcass cords deteriorate rapidly in fatigue resistance and there is a great probability or danger to cause a tire burst.